1. Field of the Invention
This invention relates to a process for the production of a molded article of a fiber-reinforced thermosetting resin, and a material for producing a molded article of a fiber-reinforced thermosetting resin composition.
The present process can be used in fields requiring the molding of large-sized articles, e.g. in the fields of automobile industry, construction industry, etc. And, the present material for producing a molded article of a fiber-reinforced thermosetting resin composition has characteristics which can provide a short cure time and impart a molded article with excellent impact resistance.
2. Prior Art of the Invention
A molded article of a fiber-reinforced thermosetting resin composition has been hitherto produced by a method of hand lay-up molding, press molding, pultrusion, filament winding molding, resin transfer molding, or the like. In the molding of large-sized articles, the hand lay-up molding method has advantages in that its mold cost is low and requires no equipment investment. However, its molding cycle is long, and since glass mats are impregnated with resin by handwork, it is difficult to increase a glass content in the molded article in view of a spring-back problem of the glass mats. In the press molding, its molding cycle is short. However, a high pressure is required to press a sheet molding compound (SMC) and a bulk molding compound (BMC) which are so thickened that they can be molded. And, the larger the intended molded article is, the higher the investment for a pressing machine and a mold therefor is. Further, the pultrusion molding and the filament winding molding have a problem in that the shape of a molded article is considerably limited.
Of the above-described methods, the resin transfer molding is the most suitable to mold large-sized articles. Further, there is also a molding method called "Structural-RIM (S-RIM) molding", in which two reactive liquids are injected into and reacted in a mold, which is preliminarily charged with a reinforcing material, to achieve higher strength by using a RIM machine used in the field of urethane. This molding method is considered to come under the category of the resin transfer molding in a broad sense in view of the molding process thereof. The conventional resin transfer molding and the S-RIM are different from each other in an injection machine.
In the resin transfer molding, due to the use of a constant-pressure injection machine, the injection time varies depending upon a back pressure which is generated depending upon the filling degree of a glass fiber in the mold. Therefore, when the glass content is increased to obtain a molded article having high strength, the injection time is considerably long, although resins having a low reaction rate and a gel time changeable with an inhibitor and catalyst system, such as an ordinary unsaturated polyester resin, can be used without problem. In a resin system using a reaction between an unsaturated polyester resin and a polyisocyanate compound to obtain a molded article having higher strength and elastic modulus, however, its reaction having a high reaction rate is involved. Therefore, if the injection takes a long period of time, the reaction proceeds to cause a high increase in viscosity during the injection, and as a result, the injection cannot be continued any longer. In such a resin system, it is therefore necessary to complete the injection for a constant and short period of time without depending upon the degree of back pressure within the mold. Since a RIM machine is an injection machine having a constant injection rate (flow rate), it hardly causes a back pressure-induced change in the injection time and is suitable for a reaction system having a high reaction rate. Even in this machine, however, when resins are injected into a mold, which is highly charged with a glass fiber, at a high rate, a considerably high back pressure is generated and sometimes surpasses pump capability to stop the machine or liquid leakage sometimes occurs in a connection portion between a mixing head and a mold.
As described above, when a molded article having a high glass content for high strength is formed, both the resin transfer molding and the S-RIM involve a back pressure problem. That is, in order to complete the molding for a short period of time, it is necessary to reduce the back pressure thereby to make it possible to inject resins smoothly, completely and promptly after the resin injection, the reaction of the resins and the curing of the resultant molded article. For this purpose, it is essential to achieve low viscosity of the resins and to keep a low viscosity by controlling a reaction rate during the injection. However, there has been no resin that can meet such requirements.